Serial device stops working when upgrade to 20.04

I have a USB device with a serial interface. It worked great on 18.04 (and on Windows) but when I upgrade to 20.04 something is interfering with communications.

When I plug in the device, it shows up as /dev/ttyACM1 or /dev/ttyACM0, which is what I expect.

When I look at /var/log/syslog I see this :

Jun 10 11:58:39 andy-HP-ZBook-15u-G3 wpa_supplicant[864]: wlp2s0: CTRL-EVENT-SIGNAL-CHANGE above=1 signal=-58 noise=9999 txrate=200000
Jun 10 11:58:39 andy-HP-ZBook-15u-G3 kernel: [  309.817011] usb 1-2: new full-speed USB device number 7 using xhci_hcd
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.967174] usb 1-2: New USB device found, idVendor=2988, idProduct=0302, bcdDevice= 2.00
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.967178] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.967180] usb 1-2: Product: 3DSystems Virtual Serial Port
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.967181] usb 1-2: Manufacturer: 3D Systems
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.967183] usb 1-2: SerialNumber: 00000000050C
Jun 10 11:58:40 andy-HP-ZBook-15u-G3 kernel: [  309.968766] cdc_acm 1-2:1.0: ttyACM0: USB ACM device

That seems ok to me.

But when I try to communicate to the device, it doesn't work. The device's communication protocol is command/response structure, so I haven't determined yet if my commands aren't reaching the device, or if the responses aren't reaching my code. (or both), but I know that if I send a command, I get no response.

Update : I've connected my hardware to a debugger and discovered that outgoing data is getting through. When I send a command from my app to the hardware, the hardware does receive the command. The problem is that when my hardware replies, my app does not receive any data.

(It's also possible that if spurious commands are being sent, it could put the device into an unexpected state.)

Things I've tried :

• Permissions : I've set the device's permissions to 777 just to be sure
• ModemManager : I've removed this package
• mtp_probe : I couldn't figure out how to disable this, so I temporarily removed the binary.

Any hints as to what's changed in serial device support from 18.04 to 20.04 would be greatly appreciated. Thank you.